1. Field of the Invention
The present invention relates to a multi-engine plant with a common freshwater cooling system, comprising several turbocharged diesel engines having a common supply pipe for coolant and a common return pipe. Some of the engines have in their coolant system a charging air cooler, a high-temperature circuit (HT) and a low-temperature circuit (LT).
2. Description of Related Art
DE-A1 32 14 855 describes the nearest prior art multi-engine plant with a common freshwater cooling system, where the individual engine in its coolant system is provided with a charging air cooler, a high-temperature circuit (HT) and a low-temperature circuit (LT). The high-temperature circuit contains a first three-way valve which is controlled by a first thermosensor and supplies a first circulation pump in the HT circuit delivery pipe to the engine with recycled coolant from the HT circuit discharge pipe from the engine and with colder coolant as needed. The low-temperature circuit contains a second three-way valve controlling by means of a second thermosensor the mixing ratio between warmer and colder coolant to the delivery pipe of the charging air cooler in the LT circuit. A supply pipe for coolant common to the engines is connected to the LT circuit, and a common return pipe is connected to the second three-way valve in this circuit.
This cooling system is designed to advantageously control the heat exchange in the charging air cooler in such a manner that the charging air in the lower operational mode of the engine is heated by the coolant and is cooled by it in the upper operational mode of the engine. The heating of the charging air at low engine load, and particularly at operation in cold surroundings where the inlet air to the compressor may be very cold, is of substantial importance to achieve good combustion conditions when the engine is run on heavy fuel oil. The heating of the charging air results in a higher compression temperature which promotes the self-ignition of the fuel. This German publication also states that the first thermosensor controls the first three-way valve so that the supply temperature of the coolant to the engine is kept constant, and that the second thermosensor controls the second three-way valve so that the discharge temperature of the coolant from the LT circuit to the common return pipe is kept constant.
The cooling system of the individual engines in the known multi-engine plant integrates the LT circuit into the HT circuit. The delivery for cold coolant to the first three-way valve is connected with the discharge pipe of the LT circuit from the charging air cooler, and the delivery for warmer coolant to the second three-way valve being connected to the discharge pipe of the HT circuit from the engine. This results in the disadvantage that the operation of the HT circuit is influenced by the operation of the LT circuit. Nor is it expedient that the inlet temperature to the cylinder cooling of the engine is kept constant, as this results in a cooling temperature for the hottest engine components which increases with the engine load.
A substantial disadvantage of the known multi-engine plant is also that the heat exchange in the charging air cooler is controlled by the second thermosensor on the basis of the water temperature in the discharge pipe of the HT circuit from the engine. This control is slow-acting in case of sudden changes in the engine load, because the load change first has to be reflected in warmer or cooler cylinder elements before the need for a changed control of coolant flows can be detected by the second thermosensor. The multi-engine plant is typically used in a ship where a number of auxiliary engines connected to power generators are in a stand-by position until the power consumption rises. This typically occurs in manoeuvring in a harbour or in other coastal areas where the starting-up of major power consumers, such as bow propellers or anchor winches, leads to a sudden jump in the power consumption. as a result a stand-by engine is started, or an engine already running is suddenly loaded at full load. The slow control of the charging air cooler involves the very well-known and undesired problem that the engine smokes heavily after the load increase until the adjustment to the new operating condition has been completed. For environmental reasons, this smoking is very undesired in coastal waters.
DE-C2 25 49 009 describes a single engine, the freshwater cooling system of which only contains an HT circuit comprising a three-way valve, which provides a circulation pump with recycled water of which a portion of the flow passes through a coolant cooler so that the delivery temperature to the engine is kept constant. The delivery pipe to the engine passes through a first intercooler. The engine also has a saltwater circuit in which sea water is pumped to a three-way valve which is controlled by the charging air condition of the engine and passes a large or small amount of cold sea water through a second intercooler, whereupon the sea water flows through the coolant cooler in the HT circuit and is passed overboard. The publication mentions the possibility of having the control of the heat exchange in the intercooler take place on the basis of the charging air pressure. It is a disadvantage in that the HT circuit contains a coolant cooler, and the cooling system is work-consuming and expensive to maintain because of the salt-water-based low-temperature circuit.
U.S. Pat. No. 5,394,854 describes a single engine with an HT circuit for cooling the engine and a first intercooler and with an LT circuit for cooling lubricant and a second intercooler. In the HT circuit, the first intercooler is inserted in parallel with the engine or coupled in series downstream of it. The second intercooler in the LT circuit is provided with a valve-controlled bypass pipe controlled by, for example, the charging air pressure. On the discharge side, each circuit has a coolant cooler provided with a valve-controlled bypass pipe, which complicates the cooling system.